Use default for Cpu struct

[emoon]

Here

https://github.com/marhel/r68k/blob/master/src/cpu/mod.rs#L11

It's possible to use #[derive(Default)]

And then it's possible to call

Rust Cpu::default()

To get all default values. Also when setting up a struct one can do this

``` Rust
Cpu {
   some_field: true,
   some_other_field: Foo,
   .. Cpu::default(), // Rest should use default settings.
}